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Indian Forest Service (Mains)

Chemistry optional

  • 2 papers
  • 200 marks each
  • 3 h per paper

Paper II covers organic chemistry and spectroscopy.

Indian Forest Service (Mains), Chemistry optional · 2022 · Paper II

Q1(e). Substituent effects on aryl negative charge

Question

Why does an ortho/para NH₂ group destabilize a negatively charged aromatic system while NO₂ stabilizes it?

Answer in brief

NH₂ donates by resonance into an electron-rich system; NO₂ withdraws and delocalizes the negative charge onto oxygen.

Key points

  • When the anionic centre is conjugated with the ring, resonance places excess electron density at the ortho and para positions.
  • An amino group contributes its nitrogen lone pair by a +M effect, reinforcing electron density rather than providing a sink for the charge.
  • A nitro group has a strong −M effect; ring electron density can be delocalized into its π-accepting system and onto oxygen.
  • These resonance effects are strongest at conjugatively connected ortho/para positions.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Model answer to Q1(e), 2022 Paper II: solution pages 5 and 6
Solution pages 5 and 6 of 34
Read the whole 2022 solution2022 question paper

Topic notes: reactive intermediates (carbocations, carbanions, radicals, carbenes, nitrenes, benzyne), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversmesomeric effect+M and −M groupsnitro group stabilisationaryl carbanionMeisenheimer complex

All parts of Question 1